A two-propeller turboprop is known, e.g. from U.S. Pat. No. 4,621,978, that comprises a turbine having two contrarotating rotors driving two respective sets of adjustable-pitch blades. The invention applies in particular to that type of airplane turboprop. Various mechanisms are known for controlling the pitch of the blades. For example, one known system comprises a conventional actuator, arranged axially in the inside space provided at the center of the turbine having an annular flow path. Mechanical connections transmit drive from the rod of the actuator radially to the adjustable-pitch blades.
The elements of those connections are complex, bulky, heavy, and expensive. In addition, the forces to be transmitted are large, thus requiring high actuating pressures for the actuator, given the necessarily limited area of the piston thereof. This high control pressure is detrimental to the lifetime of the actuator. If both sets of blades are of adjustable pitch, then the actuator is generally common to both sets.
Maintenance is made complicated by the vital components being situated inside the casing, and more particularly, by some of them being situated inside the turbine. They cannot be changed without dismantling the turbine.
The invention seeks to overcome all of those drawbacks.